The scientists who first showed that atomic number is a more fundamental property than atomic mass are Henry Moseley and Dmitri Mendeleev.
Henry Moseley, an English physicist, conducted X-ray spectroscopy experiments in the early 20th century. In 1913, he discovered that the frequencies of the characteristic X-rays emitted by elements were directly proportional to the square of their atomic numbers. Moseley's work demonstrated a clear relationship between the atomic number of an element and its position in the periodic table. This finding provided strong evidence that atomic number, rather than atomic mass, should be considered the fundamental property for organizing the elements.
Dmitri Mendeleev, a Russian chemist, is widely known for his development of the periodic table of elements in 1869. Although he arranged the elements primarily based on atomic mass, Mendeleev observed that certain elements did not fit the pattern when ordered solely by atomic mass. As a result, he left gaps in his table and predicted the existence and properties of yet-to-be-discovered elements. Mendeleev's periodic table laid the foundation for the understanding of element classification and properties. Later, when the concept of atomic number was introduced, it provided a more precise and fundamental basis for organizing the elements, validating the work of Moseley.
Together, the contributions of Henry Moseley and Dmitri Mendeleev were crucial in establishing the importance of atomic number as a fundamental property for understanding the periodicity and characteristics of the elements.